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Plate Tectonics: Further Evidence

This video segment adapted from A Science Odyssey uses animation and archival footage to provide an overview of the theory of plate tectonics. Early evidence showing striking similarities between regions on opposite sides of vast oceans suggested that in Earth's distant past what are now separate continents may once have been connected. However, this evidence said nothing about how the continents could have moved to their present positions.

This video is available in both English and Spanish audio, along with corresponding closed captions.

Like others before him, Alfred Wegener recognized an uncanny congruence in the shape of coastlines on opposite sides of the Atlantic Ocean. South America's east coast appears as if it would fit snugly against Africa's west coast -- as though one continent had been cut or torn from the other. Then Wegener stumbled upon research findings that paleontologists had used to support their conclusion that a land bridge had once connected Africa and South America. This discovery was all the inspiration Wegener needed. Just four months later, he presented his theory of continental drift, in which he concluded that identical fossilized plants and animals existed on distant continents not because land bridges connected them, but because the continents themselves were once connected.

For years, Wegener amassed a diverse collection of evidence for his theory of continental drift, which he published in 1915 in the book The Origin of Continents and Oceans and updated several times before his death in 1930. Of the many patterns he presented, Wegener showed that Europe and North America, like Africa and South America, had fossilized plants and animals in common. So, too, did Madagascar and India. Rocks along the west coast of Africa and the east coast of South America showed glacial scraping patterns that were similar to each other -- on continents that couldn't possibly sustain glaciers now. In Wegener's mind, this evidence supported the theory of continental drift irrefutably. Most other scientists in the field disagreed.

As interesting as Wegener's evidence was, it did not explain what might cause continents to move around Earth's surface. Wegener proposed two possible explanations: the first suggested that centrifugal force caused by Earth's rotation might be responsible; the second said that the gravitational forces of the Sun and Moon might have broken up and moved the continents. Both of these explanations were quickly dismissed because the forces Wegener pointed to were not nearly strong enough to move continents.

Ironically, when Arthur Holmes, one of the few proponents of Wegener's theory at the time, suggested in 1928 that convection currents inside Earth's molten mantle might be responsible for continental movement, the idea got little attention. Today, convection currents are considered the most likely explanation for the now widely accepted theory of plate tectonics.

Wegener, for all his insight, failed to see what modern plate tectonic theory explains. Continents don't plow through the ocean floor like ships in a frozen sea. A dozen major plates and several smaller plates make up our planet's crust, which comprises both the surface of the continents and the sea floor. These plates are in perpetual -- but nearly imperceptible -- motion.